Image Analysis Tools to Quantify Visual Properties of Hair Fiber Assemblies 298 another section, greater specular reflection provides more luster. Figure 1 provides an illustration of the interaction of light rays with hair. Incident light (I) can be reflected at the immediate surface by the same angle resulting in specular reflection (S f ). Some of the incident light will be transmitted into the fiber where it can experience several fates. Transmitted light can be reflected by the backside of the fiber (S r ) and be transmitted outside the fiber at a different angle than the principle specular reflection. Likewise, light reaching the surface may be reflected in a diffuse manner (D f ) due to imperfections in surface smoothness, or light that enters the fiber can interact with structural components and also undergo diffuse reflection (D r ). Some of the transmitted light may make its way through the other side of the fiber where it can interact with another fiber in much the same way as the original light ray interacts with the first fiber. In turn, light from underlying fibers may also be reflected back through the overlying fibers. Refraction: When light travels from one medium to another, the velocity of light travel can change, based on refractive index differences, causing the rays to bend or refract. The index of refraction (n) is the ratio of the speed of light in vacuum (c) to the speed of light in the material of interest (v): n = c n Eq. 1 Figure 1. Reflection of light from hair. Incident light (I) can undergo specular (S f ) or diffuse reflection (D f ) from the front-face of the fiber. It may also penetrate the fiber and undergo specular (S r ) or diffuse reflection (D r ) from the opposing side of the fiber (back-face). In less pigmented hair, light rays are able to transmit through the entire fiber and interact with underlying fibers.